Even native speakers stumble on these. The good news is they're all fixable with a few simple rules.
1. Its vs It's
Its is possessive ("the dog wagged its tail"); it's always means "it is". If you can say "it is", use the apostrophe.
2. Your vs You're
Your is possessive, you're is "you are". Same rule as above.
3. Then vs Than
Then is about time ("first we eat, then we sleep"). Than is for comparison ("this is bigger than that").
4. There, Their, They're
There = a place. Their = belonging to them. They're = they are.
5. The apostrophe disaster
Don't use an apostrophe to make a plural. "Apples for sale" not "apple's for sale".
Use a checker to stay honest
Running your text through a free grammar checker before hitting send catches the slips your eyes skip. It's the fastest way to build better habits.